1. Field of the Invention
The present invention relates to counter-flow or cross-flow heat exchangers and more particularly, but not by way of limitation to a corrugated plate exchanger which is simply constructed and efficient in operation. The present invention is particularly suited for but not limited to exchange of heat between a combustion process flue gas and an incoming fluid stream whether it be combustion air, waste gas to be incinerated or the like.
2. Description of the Prior Art
It is well known that the exchange of heat between a cold stream entering a process and a hot stream exiting a process (or vice versa) leads to a reduction in the net energy requirements of the process. Hence, it is common practice on boilers, gas turbines, waste incinerator systems and the like to use hot flue gases exiting the system to preheat the incoming combustion air and/or waste gas or liquid by means of some type of exchanger.
Also, in the cryogenic systems, such as air liquification plants, hydrogen separation plants and the like, it is common practice to exchange heat between the cold products streams and the warmer raw material feed streams to reduce the refrigeration requirements for the overall process.
Another factor effecting the cost and size of the heat exchanger is the efficiency of the heat transfer surface. An almost infinite number of combinations of heat transfer surface geometrics can be used for heat exchangers or regenerators.
One typical type of design for heat exchangers is the plate-fin exchanger wherein plates are used to separate the two fluids and the fins are used for an indirect heat transfer surface. In this type of exchanger the compressed air flows in the channel or gap between adjacent plates and the hot exhaust gases flow through the fins which are sandwiched between the two plates. The fins are attached to the plates by low resistance welding to create efficient heat transfer paths. However, in this type of construction, there is much intricate welding required to adequately connect the fins to the plates which runs the cost of manufacturing these heat exchange units extremely high.
In one designed configuration the fins referred to above are on the form of a corrugated sheet which serves to space the adjacent plates apart. Then, by a complicated system of manifolds, the cold gas and hot gas is forced to pass adjacent to one another in the alternate flow channels thus created. Again for this configuration the plates and the corrugated sheets must be metallically bonded to effect sufficient heat transfer therebetween.
This metallic bonding can then be made by either a brazing process where the entire unit and all joints are coated with a fusible brazing compound, placed in the furnace and heated to high temperatures to effect the metallic bond or each individual point of contact of metal must be hand or machine welded which has been found to be highly impractical.
In applications where large temperature differences between fluid exist, the corrugated sheet will be on the average a different temperature from the flat plate and tremendous thermal stresses will be developed and whatever bond there is between the corrugated sheet and the flat places has been found to be highly susceptible to rupture.
Other attempts have been made to more effectively transfer the heat between the fluids by means of corrugated plate exchange units such as that disclosed in the patent to D. M. Cox, U.S. Pat. No. 3,451,474, issued on June 24, 1969 and entitled "Corrugated Plate-Type Heat Exchanger." The Cox device is constructed of a plurality of folded corrugated plate units wherein the corrugations or ridges of each unit are set at an acute angle with respect to each adjacent corrugated plate. This provides each unit with a single passageway therethrough but wherein the surface walls of each passageway are corrugated to make maximum use of the effective surface for which the fluid may come into contact which provides a more efficient use than that of flat metal plates. Since the corrugations of the walls of the Cox exchanger are inclined with respect to each other, the corrugations of the two walls will abut at points where the peaks of the corrugations cross each other which will prevent collapse of the wafer under external pressure. This also provides an inlet into the wafers adjacent each end of the pack whereby one fluid may be introduced at the side adjacent one end thereof and removed near the opposite end thereof. However, the passageways for the exhaust gases constitute a plurality of plenum chambers, one in each plate unit, whereby hot gases entering therein may become dispersed toward one side or the other and not be eventually distributed throughout the exchanger. The Cox patent further teaches the introduction of the high pressure gases into the side walls at one end and removed from the opposite side walls at the diametrically opposite end of the exchanger which would tend to create a flow channel diagonally across the heat exchanger unit which would not fully utilize the counter-flow principle since dead spots or spaces would be allowed to build up on each end of the exchanger opposite the inlet and outlet ports.